The present invention relates in general to substrate manufacturing technologies and in particular to methods and apparatus for measuring relative mean plasma potential in a plasma chamber.
In the processing of a substrate, e.g., a semiconductor wafer, a MEMS device, or a glass panel such as one used in flat panel display manufacturing, plasma is often employed. As part of the processing, the substrate is divided into a plurality of dies, or rectangular areas, each of which will become an integrated circuit. The substrate is then processed in a series of steps in which materials are selectively removed (etching) and deposited (deposition) in order to form electrical components thereon.
In an example plasma process, a substrate is coated with a thin film of hardened emulsion (i.e., such as a photoresist mask) prior to etching. Areas of the hardened emulsion are then selectively removed, causing parts of the underlying layer to become exposed. The substrate is then placed in a plasma processing chamber on a substrate support structure comprising a mono-polar or bi-polar electrode, called a chuck. Appropriate etchant source gases (e.g., C4F8, C4F6, CHF3, CH2F3, CF4, CH3F, C2F4, N2, O2, Ar, Xe, He, H2, NH3, SF6, BCl3, Cl2, etc.) are then flowed into the chamber. RF energy is then applied to the chamber to form a plasma.
To avoid confusion with concepts to be presented below, all RF energy applied for the purpose of forming plasma will be referred to as “chamber RF”. The plasma created by application of chamber RF results in the formation of ionized species and also neutral molecular fragments (radical species) derived from the source gasses which are then directed toward the substrate to initiate surface chemical reactions that etch the exposed areas of the substrate. This RF energy can be applied either by capacitive or inductive means. This RF energy may be applied at one frequency or at a multiplicity of frequencies (e.g. 2 MHz, 27 MHz and 60 MHz). This RF energy gives rise to a potential within the plasma with respect to the chamber ground, commonly referred to as the “plasma potential”. In actual application, the plasma potential is time dependent in accord with the applied chamber RF power used to sustain the plasma. However we will substantially simplify our discussion, without loss of relevance, by focusing instead upon a time averaged (mean) of the plasma potential. This mean plasma potential will be denoted as Vp.
In the instance of capacitive coupling of the chamber RF to the plasma, a coupling capacitor is used. Typically this capacitor, commonly referred to as a blocking capacitor, is connected between the substrate holder and the chamber RF source. It is well known to practitioners of art that when applying chamber RF energy via this blocking capacitor, a DC voltage will develop across this blocking capacitor. Herein this voltage will be referred to as the “chamber bias voltage” (Vchamber bias). Since the capacitor is connected to the substrate holder, this chamber bias voltage also represents the voltage of the substrate holder with respect to chamber ground. The chamber bias voltage should be carefully distinguished from the “probe bias voltage” (Vb) to be defined and discussed below.
In the instance of inductive coupling of the RF energy to the chamber, the substrate holder can be considered to be at zero chamber bias voltage (Vchamber bias=0). For all RF energy coupling schemes, the chamber walls (excepting the substrate holder) will also acquire a potential with respect to chamber ground which we define as Vwall. However, for most applications of practical importance, Vwall tends to be approximately equal to the chamber ground.
Additionally, there occurs a plasma sheath layer which surrounds the entirety of the plasma and serves to separate the plasma from the chamber walls and substrate holder. This sheath layer surrounding the entirety of the plasma also contains an electric field. Charged particles transiting this sheath region will experience a force due to that field and will suffer either a net gain or loss of energy. The net energy acquired will depend on the electric potential difference and its time dependence between the substrate surface/wall and the plasma. This electric potential difference is referred to as the “sheath potential” (Vsheath).
From the foregoing discussion, one may also see that the in the instance of a plasma over the substrate holder, the sheath potential is given by the plasma potential minus the chamber bias voltage (Vchamber sheath=Vp−Vchamber bias). In the instance of a plasma over a wall surface the sheath potential is given by the plasma potential minus the wall voltage (Vchamber sheath=Vp−Vwall). Note that since the plasma potential is time dependent we may then also expect that the sheath potential will be time dependent.
The above discussion only focuses on a few parameters (e.g., sheath potential) associated with a typical plasma process. In general, the quality of the results of processes referenced above frequently depend sensitively on a number of parameters, including for example the charged species impact energy, which tends to correspond to the sheath potential since the charged species gain energy primarily while transiting the sheath above the substrate. However, a direct measurement of the sheath potential is often impractical.
Another example parameter that may also impact the process result is the rate at which ionized gas species are delivered to the substrate (i.e., ion particle flux). However, a direct measurement of the ion particle flux tends to be difficult to implement. Oftentimes, in the absence of absolute measurable values of these and other parameters, changes in these and other parameters may be yield valuable information that may also be employed to control the plasma process. Accordingly, even if absolute measurements are not possible, a detection of relative changes in the values of these and other parameters is still desirable.
In view of the foregoing, the invention presented herein discloses various theoretical bases and proposes various techniques for indirectly ascertaining the absolute values of and/or for detecting relative changes in certain parameters associated with the plasma process. The invention further discloses the use of the measurements and/or detection results in controlling various aspects of the plasma process.